The present invention relates generally to a motorgrader having a two-part articulated frame defined by a rear drive unit and a front steering unit which can be rotated or pivoted relative to the drive unit and, more particularly, to an improved method and apparatus for controlling the cross slope angle cut by such a motorgrader while the motorgrader is being operated with the steering unit in a pivoted position relative to the drive unit.
It is important to be able to grade surfaces during the construction of roadbeds, runways, parking lots and the like so that the grade and cross slope closely approximate the finished surface. In this way, the pavement is of a uniform thickness and strength. Highly skilled motorgrader operators can perform grading operations manually to produce acceptable grades and cross slopes. However, due to time pressures and the limited number of highly skilled operators, automatic control systems have been developed to assist operators and reduce the time and skill required to obtain acceptable grading.
One such system permits a motorgrader operator to preset the slope of the blade and maintain that slope by servo valves or the like activated by a blade slope sensor. The blade slope sensor is mounted on the blade or a blade supporting structure. While these systems may be made to accurately reflect the blade slope relative to horizontal, they do not always reflect the true cross slope of the cut being made by the motorgrader, i.e. the slope normal to the direction of travel of the motorgrader. For example, errors occur when the blade is not positioned perpendicular to the direction of travel which is normally the case during operation when dirt is to be moved to one side or the other of the motorgrader.
To overcome these problems, a motorgrader control system was developed to account for the angular positioning of the blade relative to the direction of travel of the motorgrader and also the inclination or slope assumed by the motorgrader. An example of such a control system is disclosed in U.S. Pat. No. 3,786,871 and is a great improvement over the previously available slope preset system. Unfortunately, the disclosed system is not effective for all operating modes of motorgraders.
Most modern motorgraders have a two-part articulated frame defined by a rear drive unit and a front steering unit which can be rotated or pivoted relative to the drive unit. Oftentimes it is required or desireable to operate a motorgrader with the front steering unit articulated at an angle relative to the rear drive unit, for example to position the drive unit on firm ground. When a motorgrader is operated in an articulated position and travelling in the direction defined by and in-line with the rear drive unit, the operating position is referred to as a "crabbed" steering position due to the movement of the motorgrader in an indirect or diagonal manner as a crab moves. For crabbed steering, the disclosed control system is ineffective.
Accordingly, there is a need for an improved method and apparatus for operating a motorgrader having a two-part articulated frame to maintain a desired cross slope when the motorgrader is operated in a crabbed steering position.